When tissue is damaged by injury, disease, etc., the morphology and function of the tissue can be restored by autonomous tissue reconstruction if that damage is of a minor extent. However, if the extent of the damage exceeds a certain limit, restoration of the original morphology and function by autonomous tissue reconstruction becomes difficult, and the tissue is left with sequelae which are changed in morphology and function. For example, while a tooth in a healthy state is held stably with its root supported by the surrounding alveolar bone, once the tooth is affected with periodontal disease, the alveolar bone is destroyed by an inflammatory reaction, and the lost part is replaced by granulation tissue.
One of the means for restoring bone tissue lost by disease etc. is the guided tissue regeneration (GTR) technique, and today this technique is commonly practiced in clinical dentistry. In the GTR technique, an isolation membrane is disposed between a tooth root and gingival soft tissue in the vicinity of the alveolar bone, which is destroyed and resorbed by being affected with periodontitis, to thereby release a space for regeneration of the alveolar bone and guide the regeneration from the remaining bone tissue.
The isolation membrane used in the GTR technique is required to have the functions of: maintaining a regeneration space by separating the gingival soft tissue and a regeneration site of the alveolar bone for a predetermined period of time according to the growth of the bone tissue; blocking entry of tissue from the gingival soft tissue to the regeneration site; and allowing permeation of nutrients, bioactive substances, etc. from the gingival soft tissue, which is rich in blood flow, to the regeneration site of the alveolar bone. In other words, an important function required of the isolation membrane is the filter effect of blocking passage of cells (serving as a barrier against cells) while allowing permeation of nutrients and bioactive substances. Thus, such an isolation membrane used for tissue regeneration medicine is called a barrier membrane.
Conventionally, barrier membranes made of a polymer material, such as polytetrafluoroethylene (PTFE), polylactide, or polyurethane, are used. A porous barrier membrane formed by sintering PTFE powder has also been put into practical use. In addition, other barrier membranes, such as a barrier membrane of polylactide formed into a non-woven fabric, and a barrier membrane of a multilayer filter composed of a spongy matrix layer made of collagen and a comparatively impermeable barrier layer, have been proposed (e.g., see Patent Literature 1, Patent Literature 2, and Patent Literature 3).
There have been two major problems pointed out with these conventional barrier membranes. The first problem is the thickness of the barrier membrane. Since the barrier membrane is implanted under the gingiva, it is required to have the physical strength to retain the membrane shape under the tissue pressure of the gingival soft tissue and maintain the regeneration space. In the case of the conventional barrier membranes made of a polymer material, the membrane thickness to meet this required physical strength is roughly 200 to 400 μm. As this thickness is equivalent to several tens of cells, implanting a barrier membrane of such a thickness under the gingiva may narrow the space for regeneration of the periodontal tissue. The second problem is the growth of bacteria inside the barrier membrane. To realize the filter function, barrier membranes made of a polymer material are in the form of a porous sintered body or fibers, and the matrix has an abundance of intricate small cavities. While cells having an approximate size of 10 μm in diameter do not enter the small cavities, bacteria not larger than one tenth of that size can easily enter the small cavities. Thus, once oral bacteria enter a part where the barrier membrane is implanted, the bacteria may grow inside the cavities, of which the number is said to be as huge as several hundred million per square centimeter, and may cause a local infection.
Recently, barrier membranes made of thin metal plate have been proposed. For example, Patent Literature 4 proposes a porous plate which is a thin metal plate having a large number of pores perforated therein by precision pressing using a micro-perforation punching die. Patent Literature 5 proposes a support for guided bone regeneration which is a thin metal plate having a large number of pores perforated therein by chemical etching processing using a photolithographic technique.